1. Field of the Invention
The present invention relates to a synchronous rectifier circuit in which the primary side is insulated from the secondary side, and having a rectifier switch formed by an FET provided on the secondary side.
2. Description of the Related Art
FIG. 2 shows a conventional synchronous rectifier circuit. SW1 indicates a main switch. SW2 indicates a rectifier switch. SW3 indicates a commutating switch. T indicates a transformer. T1 indicates a primary winding of the transformer T. T2 indicates a secondary winding of the transformer T. L indicates an output choke. C indicates a smoothing capacitor. R indicates a resistor. The switches SW1, SW2, and SW3 of this conventional example are all formed by FETs. One end of the resistor R is connected to the gate of the rectifier switch SW2. The other end of this resistor R is connected to both the output choke L and one end of the secondary winding T2.
In the conventional synchronous rectifier circuit shown in FIG. 2, the resistor R is connected to the gate of the rectifier switch SW2, and the resistor R is connected to a connecting portion connecting the secondary winding of the transformer T and the output choke L. However, when the output voltage is low and the peak value of the secondary winding voltage of the transformer T is low, the problem arises that there is insufficient voltage to drive the rectifier switch SW2.
The present invention was conceived in view of the above problem and it is an object thereof to provide a new synchronous rectifier circuit that is not affected by the voltage of the secondary winding and in which the rectifier switch operates smoothly.